Polyesters, a polyethylene terephthalate (hereinafter may be abbreviated as “PET”) in particular, are used in fields of various molded articles such as fibers, films and resins. PET is generally produced by a method of reacting dimethyl terephthalate or terephthalic acid with ethylene glycol (hereinafter may be abbreviated as “EG”) in the presence of catalysts. In the production of PET, to satisfy properties required for a particular application, functional additives such as a stabilizer, a coloring agent, an antistatic agent and an ultraviolet absorber are added. Recently in particular, when PET is used in a container for a beverage, properties such as an ultraviolet blocking property and a gas barrier property are required depending on the type of the content so as to maintain quality thereof. As measures for attaining such properties, measures such as mixing an ultraviolet absorber or a gas barrier polymer into PET and laminating thin layers of the polymer so as to form a multilayered container are taken.
Meanwhile, disposal of used polyethylene terephthalate molded articles, polyethylene terephthalate bottles (PET bottles) in particular, has become a social problem from the viewpoint of environmental destruction, and recovery and recycle of the bottles are promoted.
As a method for recycling the bottles, a so-called material recycle method has been practically used which comprises the steps of recovering post-consumer PET bottles, crushing the bottles into chips or flakes and washing and cleaning the chips or flakes so as to use them as molding raw materials for other molded articles. As for PET as molding raw materials obtained by this method, it is very difficult to prevent foreign materials such as metals, glass and resins different from PET from mixing into PET or remove the foreign materials at a low cost. The foreign materials are a problem since they cause a reduction in production efficiency in production of other molded articles, deterioration in quality of product, and an increase in production costs. Further, in the case of this method, when colored PET bottles are included in the collected PET bottles, it is expensive to separate the colored bottles from uncolored bottles, thereby causing an increase in costs of the molding raw materials. To avoid this, the colored PET bottles are no longer used, and colored labels are labeled on the bodies of transparent PET bottles instead. However, this means also has a problem that the colored labels impair the appearances of the bottles and increase the costs, whereby the effect of the colored labels is not satisfactory. Further, in the case of transparent PET bottles having other polymers contained therein or laminated thereon so as to be given functionality, it is very difficult to separate the bottles from other bottles.
Other methods for recycling PET bottles have also been studied, and one of the studied methods is a chemical recycle method. With respect to recycling of the PET bottles by this method, the present inventors have studied a method for producing high-purity bis(2-hydroxyethyl)terephthalate (hereinafter may be abbreviated as “BHET”) by depolymerizing chips or flakes obtained by crushing collected PET bottles by use of an excess of ethylene glycol and purifying the obtained decomposition product solution (depolymerization reaction product solution: EG solution). As a result, they have found and already proposed that high-purity BHET can be obtained by purifying the above reaction product solution by a combination of decolorization, deionization, crystallization, distillation, and other treatments (refer to Patent Documents 1 to 16).
(Patent Document 1)JP-A-2000-53802(Patent Document 2)JP-A-2000-169623(Patent Document 3)JP-A-2000-239233(Patent Document 4)JP-A-2001-18224(Patent Document 5)JP-A-2001-48834(Patent Document 6)JP-A-2001-48835(Patent Document 7)JP-A-2001-48836(Patent Document 8)JP-A-2001-48837(Patent Document 9)JP-A-2001-322967(Patent Document 10)JP-A-2001-335539(Patent Document 11)JP-A-2002-121173(Patent Document 12)International Publication No.01/10812 pamphlet(Patent Document 13)International Publication No.01/19775 pamphlet(Patent Document 14)International Publication No.01/29110 pamphlet(Patent Document 15)International Publication No.01/56970 pamphlet(Patent Document 16)International Publication No.02/10117 pamphlet(The term “JP-A” as used herein means an “unexamined published Japanese patent application”)
The present inventors have further studied a further improvement in purification efficiency and a further improvement in quality in this method. As a result, they have found that impurity components other than BHET, particularly a diethylene glycol (hereinafter may be abbreviated as “DEG”) and components used so as to impart functionality cause deterioration in quality of BHET. Thus, the present inventors have studied keeping the contents of these components in a product BHET as low as possible.
Since the DEG component and functionality-imparting components are contained in starting materials (PET bottles) of chemical recycle, these components must be removed efficiently with a side reaction kept from occurring.
Further, the DEG component is not only by-produced in the depolymerization step but also produced by dehydration condensation of EG during purification of the decomposition product solution, e.g., during deionization of the solution. Further, 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate (hereinafter may be referred to as “DEG ester”) produced by an ester interchange reaction between DEG and BHET also causes deterioration in the quality of BHET. However, since the characteristic of the DEG component is very close to that of EG or BHET, it is very difficult to isolate and remove the DEG component from a product BHET.
As a result of studying a method for purifying BHET efficiently, the present inventors have found that the contents of the DEG component and functionality-imparting components in a precipitate can be rendered low depending on crystallization conditions of the decomposition product solution.
Further, when BHET containing large amounts of the DEG component and the functionality-imparting components is subjected to molecular distillation, it must be allowed that the DEG component and the functionality-imparting components exist in a product BHET in large amounts or that the yield of the product is rendered low. The present inventors have found it effective for preventing this that removal of the DEG component and functionality-imparting components or prevention of by-production of the DEG component and functionality-imparting components is achieved as much as possible before the molecular distillation step.
Further, the present inventors have also found that crystallization of BHET is affected by the electric conductivity of the decomposition product solution when BHET is crystallized from the decomposition product solution and that for increasing the sizes of crystals of the precipitate so as to facilitate solid-liquid separation, it is effective to render the electric conductivity small.
The present inventors have also found that when BHET is distilled so as to be purified, a polycondensation reaction proceeds due to exposure of BHET to high temperatures, and an oligomer having a polymerization degree of 2 to 20 is by-produced.
Thus, an object of the present invention is to provide a purification method of obtaining high-purity BHET from an EG solution containing crude BHET, particularly a decomposition product solution which is obtained by decomposition of a polyester containing PET as a main component with EG and contains BHET as a main solute and EG as a main solvent, while minimizing by-production of impurity components, particularly DEG, DEG ester and oligomers, during a purification process and by separating these impurity components efficiently.
Another object of the present invention is to provide a method of crystallizing and separating diethylene glycol component and BHET from a decomposition product solution obtained by decomposition of a polyester containing PET as a main component with EG, particularly a solution which is obtained by decomposition of a polyester containing recovered PET as a main component with EG and contains BHET as a main solute and EG as a main solvent.
Another object of the present invention is to provide a method of crystallizing and separations BHET having a low content of gas barrier agent component when BHET contains the component (gas barrier agent component) derived from a gas barrier polymer.
Still another object of the present invention is to provide a method of crystallizing and separating a diethylene glycol component and BHET by subjecting a decomposition product solution obtained by decomposition of a polyester containing PET as a main component with EG, i.e., a solution containing BHET as a main solute and EG as a main solvent to deionization and crystallization treatments. Still another object of the present invention is to provide a method of crystallizing and separating BHET having a lower content of a gas barrier agent component efficiently when BHET contains the gas barrier agent component.
Still another object of the present invention is to provide a method of separating BHET having a lower content of a diethylene glycol component efficiently by subjecting an EG solution containing crude BHET, particularly a decomposition product solution obtained by decomposition of a polyester containing PET as a main component with EG, i.e., a solution containing BHET as a main solute and EG as a main solvent, to solvent removal and molecular distillation.
Still another object of the present invention is to provide a purification method of obtaining high-quality BHET efficiently by subjecting an EG solution containing crude BHET, particularly a decomposition product solution obtained by decomposition of a polyester containing PET as a main component with EG, i.e., a solution containing BHET as a main solute and EG as a main solvent, to specific crystallization and solvent removal treatments and molecular distillation.
Other objects and advantages of the present invention will become apparent from the following description.